Rear Wall Condenser For Domestic Refrigerators and Freezers

ABSTRACT

The invention relates to a condenser for refrigerators with gravity-type convection, especially for domestic refrigerators, comprising at least one tape-shaped line ( 1 ) which has a width at least twice as large as its thickness, at least two channels ( 3,4 ) separated from each other and arranged one adjacent to the other, and, if used in accordance with the invention, is installed on end and largely almost parallel to the bottom edge of the refrigerator. Line sections arranged one behind the other in the direction of flow have the top edge of the preceding line section each offset in parallel in the direction of flow relative to the bottom edge of the succeeding line section. The use of commercial extruded sectional aluminium tubes makes it possible to manufacture a condenser at reasonable cost with excellent heat exchange properties.

The invention relates to a condenser for refrigerating machines withgravity-type convection, especially domestic refrigerators, freezers orcombined refrigerators/freezers, which can be manufactured easily andcost-effectively and is characterized by a high efficiency.

Customary refrigerators, freezers or combined refrigerators/freezers areso-called compression-type refrigerators. Refrigerators workingaccording to other refrigerating principles such as absorberrefrigerators with their low noise development or thermoelectricrefrigerators allowing compact designs have the disadvantage, thoughoffering the mentioned specific beneficial features, of a considerablyhigher energy consumption and can therefore be used in marginalapplications only.

In compression-type refrigerators a compressor compresses a gaseousrefrigerant causing the latter to heat up. The refrigerant is passedthrough a condenser situated on the outside of the refrigerator. Thecondenser causes the refrigerant to discharge its heat to theenvironment and thereby to condense. Then the refrigerant flows througha throttle such as an expansion valve or capillary tube, for example,where its pressure is lowered, and into an evaporator situated in therefrigerator interior, in which the refrigerant evaporates and coolsdown heavily. The evaporator provides for an exchange of heat causingthe interior space of the refrigerator to be cooled.

The energy consumption of a compression-type refrigerator dependsgreatly on the rate at which the condenser can discharge heat to theenvironment. As energy and electric power prices are constantly on theincrease, on the one hand, and the environmental awareness of people isgrowing, on the other, it is increasingly important to manufactureenergy-saving domestic refrigerators.

The condensers used in domestic refrigerators usually comprise a tubebent in meander shape which has wire ribs welded to it for better heatdischarge. In order to increase the heat discharge by thermal radiationthe tube and the wire ribs are provided with a black paint coat.

Such condensers have the disadvantage, however, that the wire ribs willheat up to the same temperature as the tube only in the area where theyare joined to the tube. Due to the comparatively large distances betweenthe tubes and the unfavourable geometry of the wire ribs, namely theirgreat length and small diameter, larger areas of the ribs are weaklyheated only. As a result, the efficiency of the ribs is low. This leadsto a rather bad heat transfer which in the end causes the powerconsumption of the refrigerator to rise.

Other known embodiments of condensers have no wire ribs welded to thetube but a metal plate that completely covers the tube. To achieve anefficient thermal interaction between the tube and metal plate it willbe necessary however to provide as many welding points as possiblebetween the tube and metal plate. More effectively, the tube may bejoined with the metal plate by means of a continuous welding orsoldering seam. This concept provides for an improved though stilluneven temperature distribution across the heat transfer surface. Themean temperature difference between the refrigerant and the air is thuslower as is the heat transfer rate per unit of area. Anotherdisadvantage consists in the relatively high manufacturing expenditure.

Numerous concepts are known from the state-of-the-art technology, whichare aimed at improving the heat discharge and/or reduction ofmanufacturing cost of condensers for domestic refrigerators.

In EP 0 125 642 A2, for example, a back-wall condenser is proposed whichconsists only of a tube bent in meander shape with the straight tubesections having an elliptical cross section and the bent sections acircular one. Because tubes with an elliptical cross section have alarger surface area compared with those having a circular cross section,wires or metal plates are not welded to the tubes in this case. Thoughthis leads to a reduction of the manufacturing expenditure it will notimprove the efficiency of the heat exchanger.

In EP 0 843 138 A1 a back-wall condenser for domestic refrigerators isdescribed which comprises a tube bent in meander shape and clampedbetween two metal plates. One of the two metal plates is joined to athird metal plate to form a receptacle which is filled with a liquid ofhigh thermal capacity.

In CN 1616904 A a back-wall condenser is disclosed which consists of twotubes arranged horizontally one on top of the other and connected to oneanother by a multitude of tubes installed in parallel and in verticalarrangement. The vertical tubes have a diameter of 1 to 3 mm, i.e. theyare comparatively thin. The refrigerant is distributed into the verticaltubes by means of the upper horizontal tube and collected again in thebottom tube. To improve the heat discharge, a metal plate may be mountedto the tubes. The use of a multitude of thin tubes or microchannel linesmay increase the surface area and ensure an efficient heat exchange. Theuse of the thin tubes or microchannel lines will however with a view tominimizing pressure loss require the tubes to be connected in parallelwhich leads to a higher manufacturing expenditure. Another disadvantageresults from the fact that the air flows continuously in thelongitudinal direction of the tubes or microchannel lines so thatcomparatively thick laminar boundary layers can build up there whichprevent an optimum heat discharge.

Furthermore, condensers for refrigerators with forced convection areknown in which extruded sectional lines of aluminium are used with thelines being of tape shape and comprising several parallel channels.Accordingly, EP 1 557 622 A2, US 2006/0144076 A1 and DE 10 2004 024 825A1 describe condensers for forced convection with headers, and US2005/0076506 A1 and JP 06317363 condensers of this type without headers.

Due to their construction these condensers are not suitable as back-wallcondensers for refrigerators with gravity-type convection. Even if thecondensers would be installed on end and air would be allowed to flowthrough them by gravity from bottom to top, they would have an extremelybad efficiency because firstly the horizontal sectional lines wouldproduce a high flow resistance and they could secondly discharge littleheat to the environment by radiation due to their orientation.Condensers with gravity-type convection do however, unlike those withforced convection, discharge up to 45% of the heat by radiation.

It is the purpose of the invention to eliminate the disadvantages of thestate-of-the-art technology described before. In particular, it is theobjective to provide a condenser for domestic refrigerators withgravity-type convection which has a high efficiency and can bemanufactured easily and at low cost.

This problem is solved in accordance with the invention by thecharacterizing features of Claim 1; useful embodiments of the inventionare described in Claims 2 to 15.

According to the invention, the condenser for refrigerators withgravity-type convection comprises at least a line for the refrigerantwhich is formed like a tape, whose width is at least double the size ofits thickness and which incorporates at least two channels separatedfrom each other and running adjacent to one another. In the applicationaccording to the intended use the line is installed on end and largelyin parallel to the bottom edge of the refrigerator.

The air flows through the lines by gravity, i.e. from bottom to top, andthus hits the narrow edges of the lines.

In line sections arranged one behind the other in the direction of flow,with the line sections being either sections of one and the same line orsections of different lines of the condenser, the top edge of thepreceding line section is offset in parallel in the direction of flowrelative to the succeeding line section. In this way the air flow willnewly start at each line section and the insulating laminar boundarylayer at the line sections will thus be minimized in thickness. This mayincrease the efficiency of the condenser by up to 15%.

The parallel displacement of the top and bottom edges is achieved byeither arranging the line sections at an offset position in parallel inthe direction of flow or tilting them. In terms of flow conditions,effective angles of tilt should be in the range between 0° and 45° only.It is also possible to both offset and tilt the line sections which willhowever not result in a particular advantage.

In an embodiment of the condenser according to the invention, which ischaracterized by the refrigerant side being subject to a particularlylow flow resistance, the condenser comprises a vertically positionedheader at the inlet and outlet of the refrigerant. The two headers mayhave a round, namely circular, semicircular, elliptical and oval, orangular, namely triangular, square or rectangular flow cross section.The header at the inlet may incorporate a distributor.

The headers are connected primarily in parallel by a multitude of linesfor the refrigerant arranged horizontally at different vertical levelsand in parallel to one another. The headers may however also incorporatebuilt-in elements to allow all lines or several parallel lines to beconnected in series so that the refrigerant flows through the condenserin meander mode from top to bottom. The lines are installed in astraight line between the headers or are bent in wave or meander shapein horizontal direction.

It is intended to have several lines placed at a horizontal level onebeside the other and usually spaced in parallel. Preferably, the linesshould be installed with spacers such as metal plates, for example,which provides for a secondary heat exchange surface at the same time.

In another embodiment of the condenser, which is characterized by aparticularly simple construction, the line for the refrigerant is ofbent shape and placed in at least one layer formed as a plane of spiral,helicoidal, helical or three-dimensional meander contour. Headers arenot used in this concept.

The preferred three-dimensional meander-shaped layout of the line meansthat it is meander-shaped in its side view and in addition offersspatial depth. Typically, such a line is characterized in its space by ahelicoidal or helical behaviour though it does not have circular orelliptical cross-sectional surfaces as are usually found withscrews/helixes but a cross section (in top view) that is either shapedas an oblong rectangle with rounded corners or contoured like an “eight”of oblong shape.

A layer formed as a plane is understood to be a level arrangement of theline such as a three-dimensional meander, for example, which ischaracterized by spatial depth. According to the nature of theinvention, also several layers of this type may be arranged one behindthe other.

For common-type domestic refrigerators it is sufficient for thisembodiment to make use of only one line for the condenser, which ispreferably bent in three-dimensional meander shape. Such a condenser isparticularly easy to manufacture and ensures a sufficiently good heatexchange between the refrigerant and the ambient air. Specialapplications can however also be set up by means of several separatelines installed with spacers, if applicable, or lines arranged inseveral layers placed one on top of the other.

It is also possible to use lines installed by means of connectors suchas sleeves, for example. Lines of this construction may be effective interms of cost if standardized lengths of lines can be used.

To achieve large surface areas for the heat transfer, on the one hand,and to avoid larger pressure drops because of too small flow crosssections, on the other, line cross-section section areas from 3 to 30mm² are used for both the version with and the version without headerswith the hydraulic diameters of the channels being in the range from 0.1to 3 mm.

A particularly cost-effective embodiment of the condenser is achieved ifan extruded sectional tube is used for the lines. For the furtherimprovement of the heat transfer properties a rib structure may beformed on the outside of the sectional tube.

To provide a simple way of connection of the line to the domesticrefrigerator it is provided according to the invention that the line isformed with connectors at the inlet and outlet of the refrigerant withthe outlet advantageously incorporating a filter drier.

The lines are usually manufactured from aluminium because this has agood thermal conductivity, is reasonable in cost and can be easily bent.A coating with a high coefficient of emission applied to the lines willresult in an improved heat discharge by radiation to the ambient air.

In accordance with the nature of the invention, compact condensers forrefrigerators with forced convection could be made from extrudedsectional tubes which are bent so that several layers are formed instacks arranged one behind the other in the direction of flow with linesections placed one behind the other being offset relative to eachother.

In condensers with forced convection the offset arrangement could alsolead to an up to 10% higher efficiency. Another advantage results fromthe fact that the arrangement in several layers makes it possiblecompared with conventional single-layer configurations to arrive at abetter utilization of the available space.

The invention will hereinafter be described in greater detail byreference to two embodiments of condensers with gravity-type convection;wherein:

FIG. 1 schematically depicts a cross section of the line of thecondenser;

FIG. 2 is a diagram of a condenser with headers in a side view;

FIG. 3 is a diagram of a condenser with headers in a top view;

FIG. 4 is a diagram of a condenser (without headers) with a line bent inthree-dimensional meander shape.

The line 1 according to FIG. 1 consists of extruded sectional aluminiumtubes with a width of 2.54 cm and a thickness of 2.1 mm. The sides 5 ofthe sectional tubes 1 are rounded at a radius of 1 mm, for example. Toimprove the heat transfer properties, the sections are coated with blackpaint. The sectional tubes 1 are made from standardized material and canthus be provided at reasonable cost.

The line 1 has thirteen channels 3,4 through which the refrigerant 2will pass. The channels 3 have a square cross-section with roundedcorners and an edge length of approx. 1.5 mm. The two outside channels 4are of approximately rectangular shape with the outward-facing sidebeing rounded. In the present embodiment the channels 4 have a length of1.5 mm and a width of 0.5 mm.

As is evident from FIG. 2, in the condenser with header tubes 9 thelatter are connected by several pairs of lines 1 arranged in parallelone on top of the other with the pairs each placed on a horizontal planeand being separated by spacers 8 (see FIG. 3). The pairs of lines 1arranged on adjacent levels are offset relative to one another in thedirection of flow (not shown).

In the condenser with headers it is practically impossible that pressurelosses can affect the efficiency because each level has 26 channels 2running in parallel so that a very large flow cross section is providedfor the refrigerant.

In the condenser without headers (see FIG. 4) the line 1 is bent inthree-dimensional meander shape with the bending radii being approx. 1.5cm. The lines 1 placed in parallel are spaced relative to each other atapprox. 3 to 8 cm which means that they are arranged comparativelydensely (see FIG. 4). The great number of parallel channels provides alarge flow cross section for the refrigerant so that the pressure lossof the refrigerant is minimized also with a great length of the line 1.

Also in this embodiment, the line sections constituting the windings ofthe meander arranged on top of each other are offset in the direction offlow. The condenser is particularly easy and cost-effectively tomanufacture because there is no need for welding any components.

In either embodiment the planar, tape-like shape of the lines 1 andtheir dense, parallel and meander-shaped arrangement ensures a largesurface area of the condenser. The offset arrangement of the sectionaltubes 1 causes the air flow to start anew on the longitudinal sides ofall sections. As a result, the insulating laminar boundary layer at thatposition is minimized in thickness with an effective heat transfer beingensured.

The surface area of the condenser may be increased with the same tubelength by using sectional tubes 1 with ribbed structure.

The densely placed channels 3,4 and the good thermal conductivity of thealuminium will cause the total surface area of the condensers to take onthe same temperature during operation in a very short time.

The condensers can be manufactured easily and cost-effectively becausewith the extruded sectional aluminium tubes low-cost standard componentsmay be used. The condensers may be easily cleaned, a factor thatcontributes considerably to their constant performance characteristics.

Both versions of condensers can with the surface area being the sameachieve an up to 15% higher heat discharge as against conventionalcondensers. On the other hand, the surface area of the condensers couldwith the same heat transfer rate be decreased by approx. 15%.

LIST OF REFERENCE NUMBERS USED

1. line/sectional tube

2. refrigerant

3. inside channel

4. outside channel

5. rounded side

6. inlet

7. outlet

8. spacer

9. header

1. A condenser for domestic refrigerators with gravity-type convectioncomprising at least one line for the refrigerant wherein the line istape-shaped, has a width at least twice as large as its thickness,comprises at least two channels (3,4) separated from each other andarranged one adjacent to the other characterized by the line (1), ifused in accordance with the intended purpose, being installed on end andlargely almost parallel to the bottom edge of the refrigerator with thetop edge of the line sections of the same or of different lines beingeach offset in parallel relative to the bottom edge of the succeedingline section. 2-15. (canceled)